Printed circuit board having a ground plane with angled openings oriented between 30 to 60 degrees

ABSTRACT

A printed circuit board that includes conductive layers separated by insulation layers of dielectric material, at least one conductive layer being patterned and having at least one signal line embedded in an insulation material, whereby a conductive ground plan layer, separated by the insulation material and lying in a predetermined distance (d) from the at least one signal line includes a ground plane area associated to and extending along the at least one signal line, the conductive layer associated to and extending along the at least one signal line is provided with openings therein. Preferably the openings are spaces between conducting stripes, extending, seen from above, across the at least one signal line, the conducting stripes being integrally connected with the conductive remainder of the conductive layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase 35 U.S.C. §371 filing of PCTApplication No. PCT/AT2013/050238 filed on Dec. 9, 2013 which claimspriority to Chinese Application No. 201220680735.X filed on Dec. 11,2012, the disclosures of which are hereby incorporated by reference intheir entireties.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a printed circuit board (PCB) and inparticular to a printed circuit board comprising conductive layersseparated by insulation layers of dielectric material, at least oneconductive layer being patterned and having at least one signal lineembedded in an insulation material, whereby a conductive layer,separated by the insulation material and lying in a predetermineddistance from the at least one signal line includes a ground plane areaassociated to and extending along the at least one signal line.

Description of the Related Art

Increasing miniaturization and extreme electronic component density aswell as the necessity to transfer large amounts of data at high speed,e.g. at rates of 1 to 5 Gbps, can create serious problems with respectto signal integrity in PCBs. A specific problem with signal integrity ina PCB is the desire to configure signal lines with predetermined highimpedance. In order to avoid signal losses due to reflections atinterfaces with other signal lines, it is necessary to adjust the lineimpedance during the manufacture of a PCB as accurately as possible.Leakage currents, which should be as small as possible, constitute afurther problem. A still further problem, typically for high speedlines, is associated with return currents in HF-applications. Divergingreturn current paths lead to an uncontrolled impedance of thetransmission line between a signal line and a ground plane. Accordingly,it is generally recommended to avoid any splitting of the ground plane.Finally, the current paths may be considered an antenna that receivesand transmits signal energy creating electromagnetic interference.

FIG. 1 is a cross sectional view of a PCB having three conductinglayers, separated by two dielectric insulation layers. In this exampleof a conventional PCB 1 the bottom layer 2 is a structured layer ofconductive material, mostly copper, having two signal lines 3, 4.Separated by a dielectric layer 5 there is arranged a further conductivelayer 6, acting as a ground plane. This layer may be a structured layerof conductive material too, however in “electrically” proximity of thesignal lines this layer 6 is made continuous. The conductive layer isfollowed by another dielectric insulation layer 7 and here the uppermostlayer is a further conductive layer 8, which may be a structured layerof conductive material in a well-known manner. Electrical field linesare indicated schematically by broken lines.

The impedance of the signal transmission line is a function, amongstothers, of the distance between the signal lines 3, 4; ground definedprimarily by the conductive layer 6; the width of the lines 3, 4; andthe relative permittivity ∈_(r) of dielectric layer 5. At a given widthof a signal line, a higher impedance may be reached by using adielectric layer with a low relative permittivity ∈_(r) and/or byincreasing the distance between the signal line and the conductive layer6. Since the relative permittivity is determined in most cases by thecommercially available materials, such as, but not limited to prepregs,FR4, Polyimide etc., the impedance may be increased by increasing thedistance between the signal lines 3, 4 and the conductive layer 6. Theincrease in distance leads to an undesirable increase of the thicknessof the PCB as such. The current standard impedance requirement of 90-100ohms is almost impossible to achieve for a strip line with a singledielectric layer multilayer microvia stack of a HDI (High DensityInterconnect) PCB. This is a challenge for the designers, who in somecases need to introduce an additional layer just to reach the requiredimpedance in some specific tight areas of the stack. In addition, inorder to reduce loss of electrical signal in high-frequencyapplications, the PCB must show low dielectric constant and lowdielectric loss.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a PCB with signallines, having predefined impedance, which can be adjusted already duringthe manufacture of the PCB.

A further object of the present invention is to provide a PCB withsignal lines with improved signal integrity, e.g. well defined returncurrent paths.

Quite another object of the present invention is to provide a PCB withreduced problems caused by the creation of electromagnetic interference.

Another object of the invention is the provision of a PCB with signallines having, despite of small thickness, reduced loss in high-frequencyregions.

Thus, the present invention provides a printed circuit board comprisingconductive layers separated by insulation layers of dielectric material,at least one conductive layer being patterned and having at least onesignal line embedded in an insulation material, whereby a conductiveground plane layer, separated from the at least one conductive layer bythe insulation material at a predetermined distance from the at leastone signal line, includes a ground plane area associated with andextending along the at least one signal line where the conductive layerassociated with and extending along the at least one signal line isprovided with openings therein.

Preferably the openings are spaces between conducting stripes extending,as seen from above, across the at least one signal line where theconducting stripes are integrally connected with the conductiveremainder of the conductive layer.

The conducting stripes may cross the signal line at an angle of 30°-60°,preferably at an angle of 45°.

The width of the conducting stripes can be smaller than the distancebetween adjacent conducting stripes.

The ratio of the width of the conducting stripes and the distancebetween adjacent stripes is preferably between 0.1 and 0.8.

A preferred embodiment of the invention comprises at least a supportcore, a first structured conductive layer including the at least onesignal line arranged on the support core, followed by a firstprepreg-layer, a second structured conductive layer including the groundplane area layer with openings therein, followed by a secondprepreg-layer and a third conductive layer arranged on said secondprepreg layer.

A printed circuit board according to the invention may further comprisetwo differential signal lines, arranged in parallel, the ground planearea being associated with both signal lines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a PCB having two signallines according to prior art.

FIG. 2 is a schematic cross-sectional view of a PCB according to theinvention having two signal lines and a hatched ground plane.

FIG. 3 is a simplified and schematic plane view showing the arrangementof a ground plane area with respect to a single signal line.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of a PCB according to the invention will be described belowin more detail with reference to the accompanying drawings. For same orsimilar components same reference numerals are used in order to avoidredundant explanations.

A printed circuit board 9 according to the invention, as shown in FIG. 2comprises a bottom layer, namely a support core layer 10, made of adielectric material such as, but not limited to, FR 4 material orpolyimide. Conductive layer 11 is built on the support core layer 10,the conductive layer 11 being patterned and having two signal lines 12,13, extending in a direction perpendicular to the plane of FIG. 2. Thesignal lines 12, 13, for example, may serve as differential signallines, preferably with a nominal impedance of 50-100 ohm. The supportcore layer 10 with the structured conductive layer 11 is followed by afirst prepreg-layer 14, which covers the conductive layer 11. In thisway, the signal lines 12, 13 are completely embedded in dielectricmaterial, i.e. on their lower surface by the support core layer 10 andon their upper surface and their sides by the first prepreg-layer 14.

Next follows a second structured conductive layer 15 covered by a secondprepreg-layer 16. Conductive layer 15 comprises an area acting as aground plane area 17 associated with and extending along the signallines 12, 13. This ground plane area 17 of conductive layer 15 isprovided with openings 18. In the example shown, these openings arespaces between conducting stripes 19, extending, seen from above, acrosssignal lines 12, 13 where the conducting stripes being integrallyconnected with the conductive remainder 20 of conductive layer 15.

This can be seen better in FIG. 3, which is a plane view showing thearrangement of the ground plane area 17 of conductive layer 15 includingconductive remainder 20. Openings 18 and the stripes 19 are arrangedabove one single signal line 12.

Returning to FIG.2, the stripes 19 are completely embedded in dielectricmaterial, i.e. on their lower surface by the first prepreg-layer 14 andon their upper surface and their sides by the second prepreg-layer 16that also fills the openings 18.

Top layer of the circuit board 9 is a third conductive layer 21 whichmay be patterned, having conductor paths, not shown in the drawings.FIG. 2 only illustrates a part of a PCB with the features in accordancewith some embodiments of the invention. In general, PCB 9 will havelarger size and will not be limited to a certain number of layers.Furthermore, PCB 9 will have vias or microvias interconnecting conductorpaths of different layers. As shown, two electrical field lines areindicated schematically by broken lines and it should be mentioned thatparts of the conductive layer 11, adjacent to signal lines 12, 13, actpartly as a ground plane.

It has been found that a very stable performance of the strip line canbe achieved if the conducting stripes cross the signal line at an angle,α, of 30°-60°. Preferably the conducting stripes across the signal lineat an angle, α, of 45°, as depicted in FIG. 3. In this way, it ispossible to raise the impedance of the signal lines without creatingsevere problems with regard to return currents and attenuation.

It is recommendable that the width, w, of the conducting stripes 19 issmaller than the distance, s, between adjacent stripes. A preferredratio of the width, w, of the conducting stripes 19 and the distance, s,between adjacent stripes is between 0.1 and 0.8. This can be seen inFIG. 3 However, it should be noted that FIGS. 2 and 3 are only schematicrepresentations of the invention and not drawn to scale.

Support core 10 is made by impregnating reinforcing material like glassfibres with resin—e.g. epoxy resin, available under grade designationssuch as FR-4, FR-5 or others or by using polyimide resin. First andsecond prepreg-layers 14, 16 advantageously consist of FR-4, but otherdielectric materials, suitable for a lamination process may be used.

A typical thickness of conductive layers, usually consisting of copperranges between 1 and 20 μm, a typical thickness (d in FIG. 2) of thedielectric layers between 5 and 40 μm.

PCB 9 may also be a flex type PCB or a rigid-flex type PCB. In thesecases, other materials for the dielectric layers and thinner conductinglayers may be used at least for flexible parts of the PCB.

The invention is not restricted to differential signal lines and mayinclude only one line or more than two signal lines.

While the foregoing description is directed to various preferredembodiments of the invention, it should be noted that variations andmodifications will be apparent to the skilled person without departingfrom the scope of the invention as defined by the following claims.

The invention claimed is:
 1. A printed circuit board comprisingconductive layers separated by insulation layers of a dielectricmaterial, a first one of the conductive layers being patterned andhaving at least one signal line embedded in the insulation layers of thedielectric material, whereby a second one of the conductive layersseparated by at least one of the insulation layers from the first one ofthe conductive layers and lying in a predetermined distance from the atleast one signal line in the first one of the conductive layers includesa ground plane area associated with and extending along the at least onesignal line, whereby the second one of the conductive layers having theground plane area associated with and extending along the at least onesignal line is provided with openings therein, said openings beingspaces between adjacent conducting stripes that extend across the atleast one signal line, the adjacent conducting stripes being integrallyconnected with the remainder of the conductive material in the secondone of conductive layers, characterized in that the adjacent conductingstripes cross the signal line at an angle α of 30° - 60° whereby theadjacent conducting stripes provide defined return current paths; andwherein the insulation layers include a support core, a first prepreglayer and a second prepreg layer where the first one of the conductivelayers is a first structured conductive layer that includes the at leastone signal line and is arranged on the support core, the firstprepreg-layer arranged on the first one of the conductive layers wherethe second one of the conductive layers is a second structuredconductive layer that includes the ground plane area with the openingstherein and is arranged on the first prepreg-layer, and the secondprepreg-layer arranged on the second one of the conductive layers wherea third one of the conductive layers is a third conductive layerarranged on said second prepreg-layer.
 2. A printed circuit boardaccording to claim 1, characterized in that the adjacent conductingstripes cross the signal line at the angle α of 45°.
 3. A printedcircuit board according to claim 1, characterized in the each of theadjacent conducting stripes has a width that is smaller than thedistance between adjacent stripes.
 4. A printed circuit board accordingto claim 3, characterized in that the ratio of the width of the each ofthe adjacent conducting stripes and the distance between adjacentconducting stripes is between 0.1 and 0.8.
 5. A printed circuit boardaccording to claim 1, characterized in that the at least one signal linecomprises two differential signal lines, arranged in parallel with theground plane area being associated to the two differential signal lines.